The distillation of spirit is performed for the purpose of separating the alcohol more or less from the water. The boiling point of water at the ordinary standard pressures of the atmosphere, equal to 30 in. of mercury, is 212° F. (100° C), that of alcohol 173° 1°F. (78.5° C). At the sea level, the pressure of the atmosphere may frequently vary between 28.5 and 30.5 in.; the boiling points of water corresponding to these temperatures are 210° F. and 213° F. Indeed, changes in the weather may cause the boiling point of water to vary as much as 5° F. in our climate. These alterations in pressure would cause corresponding changes in the boiling point of alcohol. If we gradually raise the temperature of alcoholic fluids to a point when vapours are freely formed, it is observed that though there is a continuous absorption of heat, yet the liquid does not increase in temperature. The heat which is absorbed during the first period is doing work of a different character from that employed subsequently.
There are two phases in the process, and two different kinds of work performed by the heat employed in boiling even a kettle of water.
The first phase is indicated by a rise of temperature from 60° F. to 212° F.; the second phase by a change of state, from that of a liquid at 212° F. to a vapour at the same temperature. The quantities of heat required by different liquids in these changes varies greatly, but the variation is greatest when they pass through the second phase. Thus 1 lb. of steam at 212° F. if converted into water at 212° F., will give up heat sufficient to raise 996 lb. of water from 60° to 61° F. The heat rendered up by 1 lb. of alcohol vapour at 173° F. during condensation to liquid at 173° F. will heat 374.9 lb. of water from 60° to 61° F. These figures are sufficient to show that a small quantity of steam will boil a large quantity of alcohol. Stills of improved construction depend upon this principle.
The simplest form of distilling apparatus consists of a close vessel connected with a tube immersed in water; the tube is open, and as the vapours expand and pass into the tube, they are cooled down sufficiently to assume the liquid state, their heat being rendered up to the water. It is necessary, then, to have a large body of water to begin with, or else for the water to be continually changed. An ordinary still and worm consists of a boiling vessel with a large head for the accumulation of the bulky vapour, connected with a coiled tube immersed in a tub holding a large body of water, which is continually being changed. The coils of the tube are largest at the upper part of the worm tub, where the vapours are the most bulky. The change of water is effected in this way. A stream of cold water is conducted down to the bottom of the tube, which being there distributed, displaces the water at the top, this latter having condensed, the alcohol vapour has become warm. As the liquid below gradually makes its why upwards, it comes in contact with successive coils of tube, each one being hotter than the other, until the top coil is reached, which is at a temperature differing but little from that at which the liquid is being distilled.
The more perfect forms of stills are so designed as to take advantage of the latent heat of the vapours to raise the temperature of the liquid to be distilled.
When a mixture of alcohol and water is distilled, the liquid will not boil constantly at 173° F. until all the alcohol has passed over, but will rise in temperature gradually throughout the distillation until 212° F. has been reached. The distillate, if separated into fractions boiling between fixed points, consists of a series of mixtures of alcohol and water in definite proportions. The mixtures richest in alcohol come over first, that is to say, at the lowest temperature.
The latent heat of the vapour of a liquid with a high boiling point, can be made to boil a liquid with a lower boiling point; for instance, steam at 212° F. can boil alcohol at 173° F., and alcohol at 173° F. in turn can boil ether at 94.8° F. With a simple still, strong alcohol can be obtained from wash by repeated distillation only. Woulffe realised the fact that this wasteful and tedious process could be dispensed with, by connecting together a number of rectifying chambers in such a manner that the vapour driven off from the chamber nearest the fire should be condensed in the second, and by the heat given out by its condensation, cause the more volatile portions of the liquid of the second to distil into the third chamber, and those of the third into the fourth, and so on, until a sufficient degree of concentration is attained. Adam put this principle of rectification into practice, and it is well illustrated by a description of the working of Laugier's still. In this arrangement another process, that of dephlegmation, is put in operation, which is exactly the reverse of rectification.
It consists in partially cooling the vapours, whereby they are separated into an alcoholic and an aqueous portion; the former passes on to another cooler, only the latter being condensed. Laugier's apparatus consists of separate parts connected by the necessary pipes; these are 1st, a worm tub; 2nd, a dephlegmator; 3rd, a rectifier; 4th, a still. The rectifier is warmed by the waste heat in the flue, the still only is heated by a direct fire. The wash passes first into the worm tub, where it absorbs all the heat given off in the condensation of the alcoholic vapours, thence it passes into the dephlegmator, where it becomes heated to near its boiling point by the condensation of water vapour only; it next passes into the rectifier, where it is boiled by the aid of steam from the still; the alcohol in the steam becoming concentrated in the liquid, is boiled off; finally, the liquid in the rectifier is run into the still, from which vessel it is periodically discharged. The general arrangement is effective in separating the 2 liquids, and the former goes upwards and the latter downwards. It is not adapted to distilling mash of different kinds, nor for yielding alcohol of the highest strength at one operation.